Typically, a wafer board panel comprises layers of wood flakes or wafers formed into a composite structure using a resinous binder. The preparation of wafer board panels is complex, but broadly consists of two principal stages. The first stage comprises the preparation of the wafers and admixing thereof with the binder to form a loose layer or mat; the second stage involves subsequent compression and heating of the mat to cure the resin and form the consolidated panel.
At present, wafer board is usually manufactured in the form of planar or flat sheets. Wafer board is a recognized structural panel, finding wide application in the construction industry, particularly as a plywood substitute in residential construction.
Improvement in performance characteristics of flat wafer board panels has been attained by optimization of such parameters as wafer orientation, wafer geometry, resin selection and content, and the like.
After exhaustive optimization studies of planar wafer board it was postulated that its flexural strength characteristics could be improved if a corrugated configuration was imparted thereto. The fundamental concept of corrugating materials to thereby improve the structural properties is not a novel one. Indeed, corrugated wafer board per se has previously been manufactured in the industry. However, the wafer board panels prepared by these prior art techniques do not have the capability of economical industrial manufacture or the desired structural strength properties because they do not have a substantially uniform density.
In a recent advance, as disclosed in my U.S. Pat. No. 4,616,991, an apparatus for manufacturing a corrugated wafer board panel having a substantially uniform density was developed. The breakthrough disclosed by the patent referred to supra, resided in the apparatus being adapted to avoid having to `stretch` a planar mat into a corrugated conformation. Stretching, which had always been present in the prior art methods, would result in a final product exhibiting an uneven distribution of wood flakes and hence non-uniform density.
This prior apparatus involved a pair of opposed, spaced-apart, upper and lower platens. Each platen was formed of adjacent lengths of chain-like links. When the lengths were pushed inwardly from the side, they would shift from a planar to a sawtooth-like form. In doing so, the length of the non-undulating space between the platens in the second stage would be generally the same as the length of the planar space between the platens in the first stage. The process involved in using the apparatus was initiated by distributing a mat of loose binder-coated wood wafers between said platens. A pre-compression step was conducted by biasing the platens together, the biasing force being applied in a vertical direction, to substantially fix the wafers, thereby limiting their further movement. The platens were then biased from the side to convert them from their planar configuration to the corrugated configuration. Heat and further pressure were applied to cure the binder and produce the panel of uniform density.
The density of raw wood varies considerably. However, by the completion of the compaction and curing processes, the density of the produced wafer board will have been increased, typically by a value of about fifty percent more than that of raw wood.
It is recognized that the industry selects the density of wafer board panels so as to provide the optimum structural strength commensurate with the lowest price in terms of raw materials and manufacturing costs. So, for a typical planar (or flat) wafer board panel, its selected density would be of the order of about 640 kg/m.sup.3.
It is generally known that if one were to increase the density of a planar wafer board panel, specific material properties (namely E--modulus of elasticity, and MOR-modulus of rupture) would improve. However, such improvements would be at the expense of the `overall flexure strength` properties thereof. By overall flexure strength, bending moment capacity, load capacity, or bending strength, is meant the multiple of `S` (section modulus) and `MOR` (modulus of rupture). Additionally, it is accepted that if the density of the planar wafer board panel is increased, its `bending stiffness` will also decrease. Bending stiffness is defined as the multiple of E and I (where E is the modulus of elasticity and I is the moment of inertia).
By `high density`, `normal density`, and `low density` in the present context is meant wafer board having substantially uniform densities in the ranges of 700-900 kg/m.sup.3 ; 600-700 kg/m.sup.3 and 400-600 kg/m.sup.3 respectively.
In summary, therefore, the commonly held belief in the art was that to increase the density of a wafer board panel above the normal would result in a panel having reduced values in certain important structural properties. Such an increase in density was therefore to be avoided.